Video-based system for tennis training

ABSTRACT

The inventive system comprises a first plurality of cameras disposed around a first point in a practice area to generate a plurality of first images of an individual positioned at the first point in the course of the execution of a swing by the individual. A second plurality of cameras is disposed around a second point in a practice area, the second point being displaced from the first point. The second plurality of cameras is symmetrically positioned with respect to the first plurality of cameras about an axis defined by the position of a first theoretical line positioned between the first and second plurality of cameras, and transverse to a second theoretical line extending between the first and second plurality of cameras. The second plurality of cameras generating a plurality of second images of an individual positioned at the second point in the course of the execution of a swing by the individual. The inventive mat is placed in the practice area at a position below that damaged by the cameras, in order to guide the movements of an individual practicing tennis swings.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. ______ filed substantially concurrently with this application and directed to Video-Based Training System Incorporating Mats, the disclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

The invention relates to apparatus and methods for training athletes in the sport of tennis.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

BACKGROUND OF THE INVENTION

Tennis, as compared to other sports, presents players with a unique set of physical, temporal and mental challenges. More particularly, in tennis, the ball is in play between opposing players who are located at relatively close distances. This challenge is compounded by the speed with which a tennis ball is put into play and the range of movements available to the opposing player including direction, speed, spin and so forth. At the same time, compared to, for example, paddle tennis, the physical requirements of the sport, both in terms of strength and endurance, are substantial. All of these factors plus the rapid pace of the game underline the need for good form, which tends to conserve energy and maximize the impact of a swing, by making statistically likely to be successful movements reflexive responses which are moderated or varied in the execution to accommodate the particular objectives associated with a particular shot.

Accordingly, over the years, there has been a great emphasis on teaching proper form. Likewise, the sport has undergone considerable evolution with the introduction of new techniques, such as the two-handed backhand.

SUMMARY OF THE INVENTION

Effective tennis training, unlike training in baseball, golf, badminton and other sports using a device (such as a bat, club or racket) to hit the ball or other object in play, requires a training regimen which is highly rigorous. While existing training systems such as those disclosed by Fishman et al. (US Patent Application Publication No. US 2002/0064764 Al) which describes multimedia analysis systems utilizing at least one video camera for acquiring video data of a player performance and storage of the same followed by the storage of another performance to implement synchronization and superimposition of two sets of video data, the acquisition of data is taken from a selection of points which are not key to particular types of swings, types of players (right-handed and left-handed) and so forth. Likewise, such systems are employed in training tennis, such as the Dartfish system and software, which are used with substantially random and variable camera positions.

While prior art systems do provide the opportunity for observing a particular swing and comparing it to other swings, the present invention remedies the failure of the prior art to provide a multiple camera arrangement which can be reliably used in segmented fashion to deal with different swings, and right and left-handed players in a reproducible and effective fashion, while minimizing the amount of equipment and complexity of the method involved.

The present invention recognizes that while a single camera or set of cameras can be moved to a position where it can be used to make a video recording of player motion in the execution of any tennis swing, such an arrangement cannot, from a practical standpoint, be employed to accurately and reliably determine differences in a variety of player swings. While such prior art systems can be used to achieve desired degrees of detail, field of view and so forth, such a system cannot function to memorialize and document player performance under a wide variety of swings and conditions.

The inventive system, in contrast, will accommodate a wide range of player motions and swings for a wide variety of players and allow an exceptionally efficient and effective matching and comparison algorithm to be employed with efficiency and speed, as compared to conventional systems.

While the system of the present invention may involve the use of fixed cameras, and thus the versatility of such a system may be argued to be compromised, at least in principle, fixed cameras facilitate the achievement of performance benchmarks not reachable with conventional systems.

The inventive system comprises a first plurality of cameras disposed around a first point in a practice area to generate a plurality of first images of an individual positioned at the first point in the course of the execution of a swing by the individual. A second plurality of cameras is disposed around a second point in a practice area, the second point being displaced from the first point. The second plurality of cameras being symmetrically positioned with respect to the first plurality of cameras about an axis defined by the position of a first theoretical line positioned between the first and second plurality of cameras, and transverse to a second theoretical line extending between the first and second plurality of cameras. The second plurality of cameras generating a plurality of second images of an individual positioned at the second point in the course of the execution of a swing by the individual. The inventive mat is placed in the practice area at a position below that damaged by the cameras, in order to guide the movements of an individual practicing tennis swings.

The practice area may be a tennis court. The system may further comprise indicia for indicating the position where a person being trained is to stand. It is contemplated that the swing is associated with a range of likely trajectories for a ball being swung at and hit, the cameras in the first and second plurality of cameras being positioned outside the space defined by the likely trajectories.

The practice area may comprise a surface marked with lines defining a tennis court and a net dividing the court into two areas. The first plurality of cameras is disposed on one side of the net and the second plurality of cameras being disposed on the other side of the net.

The practice area may comprise a surface marked with lines defining a tennis court and a net. The tennis court having a real or imaginary line dividing the court into two areas. The line extending between the two areas and oriented to transverse the net. The first plurality of cameras is disposed on one side of the line. The second plurality of cameras is disposed on the other side of the line. The first plurality of cameras is disposed on the same side of the net as the second plurality of cameras.

The first plurality of cameras may comprise a camera positioned above the first point. The second plurality of cameras may comprise a camera positioned above the second point and wherein the first plurality of cameras comprises a camera positioned to the side of the first point. The second plurality of cameras may comprise a camera positioned to the side of the second point.

The first plurality of cameras may comprise a camera positioned above the first point. The plurality of cameras may comprise a camera positioned above a second point. The first plurality of cameras may comprise a camera positioned on two sides of the first point. The second plurality of cameras may comprise a camera positioned on two sides of the second point.

The cameras may comprise video cameras or still cameras.

The practice area may comprise a court surface and the points may be defined by demarcations of foot positions disposed over the court surface.

The inventive apparatus may further comprise a video recorder for recording the output of one or more of the cameras and, simultaneously or after a period of time, playing back the recorded video and/or still video images.

BRIEF DESCRIPTION THE DRAWINGS

The operation of the invention will become apparent from the following description taken in conjunction with the drawings, in which:

FIG. 1 is a flow chart generally illustrating a general implementation of the present invention; and

FIG. 2 is a plan view of an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to FIG. 1, the inventive video monitored court 10 comprises a first array 12 of cameras. Array 12 comprises cameras 14, 16, 18 and 20. The inventive video monitored court 10 also comprises a second array 22 of cameras. Array 22 comprises cameras 24, 26, 28 and 30. Cameras 14-20 and 24-30 may be of any standard quality, for example, a common no-frills video camcorder of the type conventionally sold to consumers at retail electronics outlets and typically ranging in cost from about $200-$500. However, a higher quality video camera with high resolution is preferred, mostly from an aesthetic standpoint, although high resolution will also achieve a finer comparison in evaluation of player movement. Such a comparison and evaluation of player movement is of particular value in a number of cases, for example, in the case of relatively advanced players. Such a higher level of performance is also of particular value where a player's performance at a particular point in time is being compared to earlier performance by the same player.

In accordance with the invention, array 12 is provided with a monitor 32. In similar fashion, array 22 is provided with a monitor 34. In accordance with the invention, it is further contemplated that player evaluation will be conducted while the player is on the court and that the player will be informed respecting the nature of his movement and then be allowed to, perhaps repeatedly, view that movement, sometimes overlaid with that of a standard performance done by a highly skilled player, or, at other times, overlaid with an earlier performance of the same player to evaluate changes in player movement. In accordance with the invention, it is contemplated that such changes in a player's style may be of a positive or negative nature, or may point the direction for future development.

Optionally, there may only be one monitor for the whole court placed in the corner or in the middle of the court on the side wall.

In accordance with the invention, it is contemplated that the system may incorporate more than one monitor. In this case, it may be advantageous for both monitors to be showing the same images of the individual who is being trained. In accordance with the invention, it is further contemplated that while, in principle, more than one or two images may be displayed on the screen, given the limitations and the size of the screen, sending two images to the screen at one time offers good imaging of the person practicing. Likewise, if desired, a single image may be used from any one of the eight cameras in the system. The provision of images to a monitor is largely a function of obtaining a relatively comprehensive picture of good quality subject to user preferences. Likewise, as larger screens become economically available, more than two images may be displayed simultaneously with high quality. Finally, a single monitor may be used. Unlike the cameras, it is contemplated that the monitor or monitors may be moved to accommodate easy viewing during the employment of the invention in a teaching exercise. For example, a monitor may be positioned where the student is naturally facing while performing a training exercise.

While a wide range of monitor sizes may usefully be employed, in accordance with the present invention, it is contemplated that relatively large monitors are to be employed. Monitors 32 and 34 may, for example, have a standard diagonal measurement of 42 inches (or smaller but preferably larger) and are sufficiently large for a player to easily enough view movements and receive instruction while he is using the inventive video monitored court 10.

Similarly, a wide range of monitor technologies may be employed. More particularly, monitors 32 and 34 may use, for example, plasma technology. Generally, currently plasma technology is believed by the inventor to be of superior value in the implementation of the invention, because of the very large screen sizes that can be achieved by plasma technology. However, given that liquid crystal technology can produce screens in the 46 inch (115 centimeters) range, it is expected that the brighter output or other advantages of liquid crystal screens (and perhaps other technologies) will make them increasingly effective, as the technology is implemented in larger size and brighter displays.

It is also preferred that monitors 32 and 34 be protected from mechanical impacts, for example, tennis balls which stray to the sides of court 10. Video monitors 32 and 34 may be housed in a protective cage made of clear transparent plastic, heavy gauge wire or metal rod stock. Alternatively, a shield of clear transparent plastic, for example plastic having a thickness of a 1.25 cm, may be used to protect the monitors.

Bright displays are of value because tennis is often played under bright lighting conditions, including direct sunlight. However, in accordance with the invention, it is contemplated that the inventive video monitored court 10 most often would be deployed in an indoor configuration. However, the invention is equally applicable to deployment outdoors.

The configuration of the inventive system for two trainee positions 36 and 38 is illustrated in FIG. 1. The inventive arrangement of cameras is deployed on a tennis court of conventional dimension.

Camera 14 is positioned at a distance 44 of about, for example, 18 feet in front of net 46 and a distance 48 of, for example, 6-12 feet to the left of length 50 (the “doubles line”), of the court. Camera 14 is positioned at a height of roughly, for example, about 78 inches above the surface of the court (although a wide range of heights will work adequately) and aims, for example, at the net at an area adjacent and on the same side of the net as a person being trained, at a point approximately 34 inches above the surface of the court, roughly corresponding to a point approximately at the waist of the person using the inventive court 10. Such a person working at the net may be working on his transition game or his volleys and overheads.

Camera 16 may be positioned at a distance 52 of one foot in front of the baseline or width 54 of the court and a distance 56 of about six to twelve feet, for example, to the left of length 50 of the court. Camera 16 is positioned at a height of roughly about 78 inches above the surface of the court and aims generally at player position 36 at a point approximately three feet above the surface of the court, roughly corresponding to a point approximately at the waist of the person using the inventive court 10. In accordance with the invention, it is contemplated that the player will be centered in the center of the monitor. More particularly, it is contemplated that the monitor will display the player centered on the screen with an area above and below the player. The area above the player visible on the monitor will be about four or five feet above the player, being generally determined by the height needed to fully show the motion of the server and his racquet, as well as the path of the ball. The area below the player on the screen may be relatively small but large enough to ensure that during the entire movement the feet of the player are seen on the screen.

Camera 18 is positioned at a distance 58 of, for example, roughly 15 to 25 feet in front of baseline 42 at a distance 60 of, for example, roughly ten to fourteen feet to the right of length 50 of the court. Camera 18 is positioned at a height of, for example, eight to twelve feet above the surface of the court and aims directly at player position 36, centered, for example, at a height, for example, roughly about three feet above the surface of the court where the player is standing, roughly corresponding to a point approximately at the waist of the person using the inventive court 10. In accordance with the invention, it is contemplated that the cameras will use wide angle lenses, although narrower lenses, for example lenses having a field of view of approximately 40° or 80°, for example may be used. This means that the viewing area associated with, for example, camera 28 would vary from approximately twice the angular width 59 of the student to the full angular width 61 of the court baseline. Likewise, camera 26 is selected to have a field of view which encompasses expected ranges of position for the player during the particular exercise.

Camera 20 is positioned at a distance 62 of roughly about 35 to 37 feet in front of net 46 and a distance 64 of roughly about 10 to 14 feet to the right of length 50 of the court. Camera 20 is positioned at a height of roughly about 20 to 30 feet above the surface of the court, as may be permitted by the height of the ceiling of the facility, and aims directly down at player position 36. Alternatively, it may be aimed directly across to the opposing side and the position 38 occupied by a student on the other side of the net and have an angular width 63. In accordance with the invention, it is contemplated that the mat will be put in the illustrated position in the case of instruction of younger players. In the case of more advanced players, it is expected that the mat will be put outside the baseline of the court, typically in a position where a player is located when hitting groundstrokes during a game. Moreover, the mat is less relevant, if at all, to use during the training of high performance high level players.

Optionally, the inventive video-monitor court 10 may be provided with an area demarcation 66 which may be tape, paint or other indicator applied to the surface of the court. Another option is for area demarcation 66 to take the form of a mat with a plurality of foot position indicators which are used to guide the student being trained in the execution of particular shots. Such mats are described in the above-referenced co-pending related applications, the disclosures of which are hereby incorporated herein by reference.

However, there is no need to have a student limited to practicing at a single position since a wide angle lens will be able to capture a large area and record the same. In this regard overlays of images may be varied, in position to correct for positional variation, for example, when comparing two performances.

Cameras 24, 26, 28 and 30 are associated with the area demarcation 68, which may be a simple rectangle or a mat or footprint demarcations printed or adhered to the surface of the court, as may be the case with respect to area demarcation 66. If a mat is not being used the student can simply locate himself looking in the monitors, however it is preferred thatt a marking be included in the inventive court in order to have uniformity in the display of players by using the same player position and same camera position. Area demarcation 68 provides an optimized viewing position for certain swings with respect to which training may not be appropriate with the player position over area demarcation 66. Accordingly, as we discussed in detail below, area demarcation 66 and 68 are particularly valuable in connection with training of certain shots which training depends upon whether the student being trained is right-hand dominant or left-hand dominant. The result is a highly efficient and effective training method.

Camera 24 may have an angular field of view 67, while camera 26 may have an angular field of view 69.

It is also noted that an arrangement of the inventive system with both positions on the same side of the net as illustrated in FIG. 2, where like numerals designate analogous elements may be used.

Returning to FIG. 1, the positions of cameras 24, 26, 28 and 30 are disposed in a mirror image with respect to cameras 14, 16, 18 and 20, respectively, and are symmetrical about net 46. Cameras 24-30 are positioned at the same height as cameras 14-20 respectively and are also aimed a little above the waist of the player. For example, camera 24 is positioned at a distance about, for example, 18 feet in front of net 46 and a distance of about, for example, 6 to 12 feet to the right of the doubles line of the court. Camera 24 is positioned at a height of 78 inches above the surface of the court and aims perpendicular to the doubles line to, for example, photograph a player standing in front of it and in its field of view at a point approximately three feet above the surface of the court, roughly corresponding to a point approximately at the waist of the person using the inventive court 10. In accordance with the invention, it is also contemplated that camera 30 may be used to video a player in position 38, as a variation of the embodiment disclosed above, if that is desired by the individual using the system.

During use, the student is positioned, for example for the case of a young student, in position 36 or 38, as is appropriate. More advanced players may be positioned further from the net. The student may throw a ball up in the air or bounce a ball to allow the student to practice a shot during the videoing of the student's movement. Alternately, a ball pitching machine may be used. After the student has practiced swinging a tennis racket, an instant replay may occur on the monitor that is closest to the student in order for her to see her performance and receive guidance from the instructor.

In accordance with a preferred embodiment of the invention, a tennis player, for example a professional tennis player whose form is to be emulated, performs swings and is videoed in the inventive court 10. Such videos would be generated for a plurality of different swings.

After a student being trained performs a particular swing, the video of the professional player doing the same swing is played, optionally superimposed over and synchronized to the swing of the student. This enables the student to see differences and perhaps better understand how to improve performance.

In accordance with the invention, it is contemplated that particular advantage can be achieved by the generation of mirror images of the performance of the swing by the professional player. For example if a student being trained is left-hand dominant and the professional player is right-hand dominant, a mirror image video of the professional player may be superimposed on the video of the student's swing.

Optionally, after the student being trained performs a particular swing, a videotape of that student's own swing can be played and superimposed to show changes in the style of the student, perhaps good changes or perhaps changes which represent a deterioration in performance.

In accordance with the invention, it is also contemplated that a trainer may choose to show the video to the student prior to the student's execution of a particular swing in order to provide some general guidance to the student. In such event, the use of mirror images of players of opposite hand dominance may be used, and the proper orientation of the same provides a lower mental overhead and thus ease of use as compared to merely looking at a video of a professional player to be emulated.

In all of the above examples, the uniformity provided by having a fixed position of play and fixed cameras, which are fixed during the time that the student is using the inventive court 10 and are in the same positions when the professional player to be emulated is generating the training videos used in the system, provides significant advantages. Likewise, the student at an earlier point in time would have assumed the same position facilitating assessment of his swing at a later point in time. Uniformity in student and player position can be achieved by placing a player position marking on the court.

Likewise, because the system may be used by both right hand dominant and left-hand dominant players, the provision of symmetrical camera arrangements allows the transposition of left-handed training to right-handed students and vice versa.

Moreover, enhanced accuracy in the inventive system may be achieved through the use of mats which include demarcations, for example printed demarcations, indicating positions for the feet showing various portions of particular tennis swings.

In accordance with the invention, it is contemplated that certain swings would be performed in different positions depending upon the particular swing being practiced and whether the individual is right-hand or left-hand dominant.

For example, player position 36 would be appropriate for filming of both students being trained and professionals to be emulated when performing a backhand, a backhand volley, backhand overhead, and a backhand approach shot, provided that the players are right-hand dominant.

Conversely, player position 38, for right-hand dominant students being trained and right-hand dominant professionals to be emulated, is appropriate for training an overhead swing, a forehand volley, a forehand approach shot, a serve, and a forehand. During filming and training, they should be positioned, for example, over demarcation area 68 or other positions as appropriate for the swing being practiced.

For left-hand dominant players, who should be positioned, for example, over a demarcation area 66, player position 36 is appropriate for filming of both students being trained and professionals to be emulated when performing a forehand, a serve, an overhead, a forehand approach swing and a forehand volley.

Conversely, left-hand dominant players may be trained at player position 38 for a backhand swing, a backhand overhead, a backhand approach swing, and a backhand volley.

In accordance with the invention, it is contemplated that the positions of the cameras may be controlled electronically, for example, in response to a remote control or other console. Alternatively, software may be used to control camera orientation to automatically determine proper viewing angle and zoom setting (i.e. focal length).

It is also noted that the control of the cameras may be varied between an adult and a junior orientation, or perhaps an adult, junior and young junior orientations.

The possibility also exists to use artificial intelligence software to convert existing images to images standardized to the system, thus allowing comparison with players who have not performed at the standardized inventive court.

Similarly, artificial intelligence software may be provided to adjust the position, orientation and zoom setting of the cameras to emulate the position, orientation and focal length of a camera with which existing footage was taken. In connection with this, the positions of the cameras including their position, orientation and focal length may be computer-controlled using a robotic arm, track system or other mechanical artifice.

In accordance with the invention it is also contemplated that the position of the cameras may be varied by computer during the practicing of different swings, with the objective of optimizing the display of the position of the person being videoed, but at the same time keeping the position of the cameras during the reference video, whether it be that of a professional to be emulated, or the person being trained at another time. Such camera positions may be stored in memory to allow maximum flexibility in the system and the ready use of prerecorded swings.

The use of the inventive video monitored tennis training court 10 may be better understood with reference to its use by a right-handed player practicing a backhand swing.

It is also contemplated in accordance with the invention that the video monitors may be employed in a so-called “split screen” mode, with simultaneous display of different views of the individual being trained. For example, one camera may be directed at the feet of the individual while another camera may be directed at the torso of the individual.

It is also contemplated that mats having foot positions indicated may be used, and also that the mat may be replaced with a video screen with a moving or static image of a marking or indicator indicating the position of the feet of the user during the execution of a particular swing. In accordance with this embodiment of the invention, it is contemplated that the individual receiving instruction would move his feet, keeping each of his feet over the indication for that particular foot for the duration of the swing. At the same time the video monitor can display to the individual being trained the accuracy with which he is following the moving indication of his foot position as his foot, in order to keep his feet along the course in which each of his feet should be moving during the execution of the particular swing. It is also contemplated that the moving positions of the markings may be displayed during an initial period in which the student observes and studies that movement, perhaps without moving his feet. Alternatively, the student may stand at a position beside the video display of the markings and move along with the display without putting his feet over the markings, optionally while a video recording is being made.

While various modifications will be obvious to those of ordinary skill in the art, it is contemplated that such modifications are within the spirit and scope of the invention which is defined and limited only by the appended claims. 

1. Apparatus for training an individual to play tennis, comprising: (a) a first plurality of cameras disposed around a first point in a practice area to generate a plurality of first images of an individual positioned at said first point in the course of the execution of a swing by said individual; and (b) a second plurality of cameras disposed around a second point in a practice area, said second point being displaced from said first point, said second plurality of cameras being symmetrically positioned with respect to said first plurality of cameras about an axis defined by the position of a first theoretical line positioned between said first and second plurality of cameras, and transverse to a second theoretical line extending between said first and second plurality of cameras, said second plurality of cameras generating a plurality of second images of an individual positioned at said second point in the course of the execution of a swing by said individual.
 2. Apparatus as in claim 1, wherein the practice area is a tennis court.
 3. Apparatus as in claim 1, further comprising indicia for indicating the position where a person being trained is to stand.
 4. Apparatus as in claim 1, wherein said swing is associated with a range of likely trajectories for a ball being swung at and hit, said cameras in said first and second plurality of cameras being positioned outside the space defined by said likely trajectories.
 5. Apparatus as in claim 1, wherein said practice area comprises a surface marked with lines defining a tennis court and a net dividing the court into two areas, said first plurality of cameras being disposed on one side of said net and said second plurality of cameras being disposed on the other side of said net.
 6. Apparatus as in claim 1, wherein said practice area comprises a surface marked with lines defining a tennis court and a net, said tennis court having a real or imaginary line dividing the court into two areas, said line extending between the two areas and oriented to transverse the net, said first plurality of cameras being disposed on one side of said line and said second plurality of cameras being disposed on the other side of said line, said first plurality of cameras being disposed on the same side of said net as said second plurality of cameras.
 7. Apparatus as in claim 1, wherein said first plurality of cameras comprises a camera positioned above said first point and said second plurality of cameras comprises a camera positioned above said second point and wherein said first plurality of cameras comprises a camera positioned to the side of said first point and said second plurality of cameras comprises a camera positioned to the side of said second point.
 8. Apparatus as in claim 7, wherein said first plurality of cameras comprises a camera positioned above said first point and said second plurality of cameras comprises a camera positioned above said second point and wherein said first plurality of cameras comprises a camera positioned on two sides of said first point and said second plurality of cameras comprises a camera positioned on two sides of said second point.
 9. Apparatus as in claim 1, wherein said cameras comprise video cameras.
 10. Apparatus as in claim 9, further comprising still cameras.
 11. Apparatus as in claim 1, wherein said cameras comprise still cameras.
 12. Apparatus as in claim 1, wherein said practice area comprises a court surface and said points are defined by demarcations of foot positions disposed over said court surface.
 13. Apparatus as in claim 1, further comprising a video recorder for recording the output of one or more of said cameras and, simultaneously or after a period of time, playing back said recorded video.
 14. Apparatus as in claim 13, wherein said practice area comprises a surface marked with lines defining a tennis court and a net, said tennis court having a real or imaginary line dividing the court into two areas, said line extending between the two areas and oriented to transverse the net, said first plurality of cameras being disposed on one side of said line and said second plurality of cameras being disposed on the other side of said line, said first plurality of cameras being disposed on the same side of said net as said second plurality of cameras.
 15. Apparatus as in claim 14, wherein said first plurality of cameras comprises a camera positioned above said first point and said second plurality of cameras comprises a camera positioned above said second point and wherein said first plurality of cameras comprises a camera positioned to the side of said first point and said second plurality of cameras comprises a camera positioned to the side of said second point.
 16. Apparatus as in claim 3, wherein said markings are provided by an electronic display.
 17. A method using the apparatus of claim 16, wherein a user stands apart from the display and follows the movement of the display with his feet. 